The invention relates to an incandescent plug for air-compressing internal combustion engines having a plug body with a connecting device mounted on the plug housing for receiving heating current, an incandescent tube that is mounted on the plug housing and which is closed at an end opposited the plug housing, and a wire filament-like resistance element embedded within an electrically insulating material within the incandescent tube.
Measurements conducted on diesel-engined vehicles have shown that in some running conditions, the combustion chamber temperature and therefore the incandescent tube temperature of unheated (currentless) incandescent plugs (glow plugs) is approx. 400.degree. to 500.degree. C. Since misfire-free operation is only achieved at a temperature above approx. 850.degree. C., these running conditions are accompanied by poor exhaust gas and noise behaviour. It is therefore expedient to allow the incandescent plugs to be switched on at least periodically.
In the case of the known rod-shaped incandescent plugs of the type mentioned at the outset U.S. Pat. No. 4,556,781, (German Offenlequngsschriff 38 25 013), the filament-like resistance element extends over the total length of the incandescent tube. These known rod-shaped incandescent plugs require for a constant temperature of approx. 900.degree. to 1000.degree. C. and an electrical output of more than 120 W per plug in still air.
Such a high electrical output is not available for continuous operation which is why a known incandescent plug of this type is a failure as a continuous incandescent ignition energiser.